Anyone who has ever run a marathon or watched a friend or family member train for the 26.2-mile event knows the concerns that trouble competitors. Will the weeks of preparation pay off? Might a niggling injury or a bout of bad weather intervene? The runners gearing up for the Virgin London Marathon this weekend face deeper worries. The stark images of the twin bomb attacks on the Boston Marathon on April 15 are seared into the memory.

On Friday, as authorities in Boston locked down the city in pursuit of at least one suspect, London’s Metropolitan Police and marathon organizers continued working on the final details of beefed-up security plans designed to ensure that the anticipated contingents of 37,000 competitors and hundreds of thousands of spectators were safe. At a press conference outside Scotland Yard, Julia Pendry, the Met’s chief superintendent, who has worked on the security operation of the London Marathon for the past five years, said police have reviewed their security strategy and will increase the number of on-duty officers by 40% over last year. (Officials are not revealing the total number of police expected to be deployed.)

“Following the terrible events in Boston, we are providing additional visible reassurance to the public in what is naturally a worrying time,” she told reporters. “I would stress there is no change to the threat level to London, and nothing at this stage to link the Boston bombings to the London Marathon. Nevertheless, we want to do all we can to help provide a secure environment in which the runners, spectators and volunteers can enjoy themselves.”

The Boston attacks may have raised tensions, but London authorities had already planned for the possibility of their marathon’s being a target. The city has weathered terrorist attacks by Irish Republican groups and al-Qaeda-inspired terrorists. Four suicide bombers in July 2005 killed themselves and 52 commuters by detonating backpack bombs on buses and underground trains. Concerns about the threat to last summer’s Olympic Games saw an additional 12,500 police officers rostered; the Games took place without incident. Just this week, the funeral of Margaret Thatcher provided another tough test for London’s ability to ensure the security of large-scale events. More than 4,000 police officers monitored the crowds lining the streets and encircling St. Paul’s Cathedral as Queen Elizabeth II, 11 serving Prime Ministers and 17 serving foreign ministers from around the world attended the service.

This year’s marathon will be London’s 33rd, and months of preparation have gone into organizing the event that is expected to include elite runners such as Olympian Mo Farah, world-record holder Patrick Makau and defending London Marathon champion Wilson Kipsang. Runners will follow a route that winds past some of London’s best-known landmarks, including Tower Bridge and Buckingham Palace. And despite heightened security fears, Prince Harry is still set to hand out medals to winners at the finish line.

The organizers acknowledged that Boston has made this a nervy occasion. “When you see something like what happened in Boston, you have to take account of it and say, ‘Is there more that we can do?’ ” Nick Bitel, CEO of the London Marathon, said in a press conference on Tuesday. “We’re confident that we’re taking all reasonable and appropriate measures to ensure that the race is safe for spectators, for runners and for the people of London.”

Marathon organizers also took that message directly to the runners. Gary Long, 33, a stay-at-home dad from Doncaster and a first-time marathoner, says one of his first questions upon hearing about Boston was what the attacks would mean for London’s race. “I think it was difficult not to, really, with [Boston] being so [recent] and both being major world marathons,” he says. He was reassured by an e-mail from the London Marathon organizers. “I was of the mind-set that if I was told it was safe to run, then I’d run.”

However, Long’s wife Rachel and his 2-year-old son Callan won’t be on the sidelines cheering him on. Long says the couple made the decision months ago because of Callan’s young age but adds that the news from Boston cemented the choice. “If I had a lot of support coming down, I might be a bit more wary, because of the supporters that were injured on Monday,” he says. “It’s a long way to be running when you’re worrying about someone else.”

Lucy Burton from Hampshire is another first-time marathoner. Though she has been training for months and raised more than £3,000 for the charity Children with Cancer U.K., she briefly considered dropping out. “I thought, Who did the Boston bombing? And then, Is that going to affect Sunday? Is this going to happen again?”

But Burton, who will turn 25 the day before the race, says she will now run with a greater sense of purpose. “What’s happened on Monday has made me even more determined to cross that finish line now [and] to show support for the people who were injured and lost their lives.”